DISTRIBUTION OF ANTARCTIC MACROPLANKTON 157 



inclination to follow a rich macroplankton unless there is also a rich development of 

 " krill", but it is quite possible that the hydrological conditions which give rise to a rich 

 macroplankton are also in certain circumstances the conditions which provide the 

 environment which the whales seek in the Antarctic. 



It is obvious enough that the occurrence of whales does not vary regularly with the 

 quantity of macroplankton. The Bransfield Strait, for instance, is normally exceedingly 

 poor in plankton, and yet it is a region in which whales are known to be plentiful. We 

 have already seen, however, that there must be more than one factor controlling the 

 distribution of whales in the Antarctic, and consequently it cannot be expected that one 

 set of circumstances will determine that distribution at all times and places. 



SUMMARY 



This paper is concerned with the plankton samples taken in Antarctic surface waters 

 with the 1 m. oblique nets, and its object is to describe the horizontal distribution of the 

 macroplankton as a whole. It is based on the analysis of about 600 samples collected in 

 the period 1927-31. 



The Antarctic convergence is the northern boundary of the area under consideration. 

 Some Antarctic species are as common in the sub-Antarctic water on the north side of 

 the convergence as they are in the Antarctic water on the south side, some occasionally 

 stray into the sub-Antarctic water, and others are entirely confined to the south side of 

 the convergence. 



The amount of diurnal variation shown by each species has been estimated, and the 

 average number per haul at different times of day and night plotted on graphs. It is 

 found that there is every gradation between the species which can be caught in large 

 numbers in the middle of the night but which are entirely absent in the middle of the 

 day, and those which are taken equally at any time of day or night, or may even be 

 commoner during the day. A period of daytime hours is allotted to each of the more 

 variable species, and samples taken within that period are not regarded as valid in- 

 dications of the presence or absence of the species in question. 



Brief notes are given on the individual distribution of each species, and it is shown 

 that some are typical of the colder water or high latitudes, some characterize the warmer 

 water or lower latitudes, and others again may be taken equally in warm or cold regions. 



The richness of the plankton varies greatly in different places and at different times. 

 In the neighbourhood of the South Orkneys and South Shetlands, and in the eastern 

 part of the Bellingshausen Sea the plankton appears always to be very scarce, possibly 

 as a result of the upwelling of deep water. In other regions it is variable but generally 

 very rich except during the winter months. It is possible that the boundary between 

 the scarce plankton near the South Shetland Islands and the richer plankton of the 

 Drake Passage may shift southwards in the latter part of the summer. 



The distribution of species typical of warm or cold water is specially interesting. 

 There is abundant evidence to show that in a given area the proportion of cold-water 

 species in the plankton population becomes reduced as the summer goes on. There is 



